Datasheet LTC3605 (Analog Devices) - 8

LTC3605
operaTion Main Control Loop
point. Continuous operation is forced during OV and UV The LTC3605 is a current mode monolithic step-down condition except during start-up when the TRACK pin is regulator. In normal operation, the internal top power ramping up to 0.6V. MOSFET is turned on for a fixed interval determined by Foldback current limiting is provided if the output is a one-shot timer, OST. When the top power MOSFET shorted to ground. As VFB drops to zero, the maximum turns off, the bottom power MOSFET turns on until the sense voltage allowed across the bottom power MOSFET current comparator, ICMP , trips, restarting the one-shot is lowered to approximately 40% of the original value to timer and initiating the next cycle. Inductor current is de- reduce the inductor valley current. termined by sensing the voltage drop across the bottom Pulling the RUN pin to ground forces the LTC3605 into power MOSFET ’s VDS. The voltage on the ITH pin sets its shutdown state, turning off both power MOSFETs and the comparator threshold corresponding to the inductor most of its internal control circuitry. Bringing the RUN pin valley current. The error amplifier, EA, adjusts this ITH above 0.7V turns on the internal reference only, while still voltage by comparing the feedback signal, VFB, from the keeping the power MOSFETs off. Further increasing the output voltage with that of an internal 0.6V reference. If RUN voltage above 1.2V turns on the entire chip. the load current increases, it causes a drop in the feedback voltage relative to the internal reference. The ITH voltage
INTV
then rises until the average inductor current matches that
CC Regulator
of the load current. An internal low dropout (LDO) regulator produces the 3.3V supply that powers the drivers and the internal bias At low load current, the inductor current can drop to zero circuitry. The INTV and become negative. This is detected by current reversal CC can supply up to 100mA RMS and must be bypassed to ground with a minimum of 1µF comparator, IREV , which then shuts off the bottom power ceramic capacitor. Good bypassing is necessary to supply MOSFET, resulting in discontinuous operation. Both power the high transient currents required by the power MOSFET MOSFETs will remain off with the output capacitor supplying gate drivers. Applications with high input voltage and high the load current until the ITH voltage rises above the zero switching frequency will increase die temperature because current level (0.6V) to initiate another cycle. Discontinu- of the higher power dissipation across the LDO. Connect- ous mode operation is disabled by tying the MODE pin to ing a load to the INTV INTV CC pin is not recommended since CC, which forces continuous synchronous operation it will further push the LDO into its RMS current rating regardless of output load. while increasing power dissipation and die temperature. The operating frequency is determined by the value of the R
V
T resistor, which programs the current for the internal
IN Overvoltage Protection
oscillator. An internal phase-lock loop servos the oscillator In order to protect the internal power MOSFET devices frequency to an external clock signal if one is present on against transient voltage spikes, the LTC3605 constantly the CLKIN pin. Another internal phase-lock loop servos the monitors the VIN pin for an overvoltage condition. When switching regulator on-time to track the internal oscillator VIN rises above 17V, the regulator suspends operation by to force constant switching frequency. shutting off both power MOSFETs. Once VIN drops below Overvoltage and undervoltage comparators OV and UV 15V, the regulator immediately resumes normal operation. pull the PGOOD output low if the output feedback volt- The regulator does not execute its soft-start function when age, V exiting an overvoltage condition. FB, exits a ±10% window around the regulation 3605fh 8 For more information www.linear.com/LTC3605 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Description Revision History Typical Application Related Parts